This invention relates to a method and device for effectively shot-peening curburized products such as gears, etc.
Generally known has been a shot-peening processing in which particulate shot are projected on the surface of the part that receives the concentration of stress of the curburized products such as gears, etc., to increase the compressive residual stress and improve the fatigue strength. Japanese Patent Early-publication Nos. 60-150966 and 61-265271 suggest that a double shot-peening processing be conducted to increase the effect of a shot-peening processing.
In the double shot peening, at the first stage a deep (thick) compressive residual stress layer is formed in a product (the broken line in the graph of FIG. 2) by applying to a product a shot-peening processing with large-diameter particles, and at the second stage a high compressive residual stress is obtained in the top of the surface layer in the product by applying to the product to a shot-peening processing with small-diameter particles (the thick, continuous line in the graph of FIG. 2). By applying such a double shot-peening processing, a compressive residual stress as shown by the broken line in the graph of FIG. 1 is obtained.
However, since the above double shot-peening processing has two processing steps, it is time-consuming and thus the efficiency of processing worsens. Also, it requires equipment of two devices, one of which is used for projecting the large-diameter particles and the other of which is used for projecting the small-diameter particles. Thus it has a problem, in that the cost of equipment is high.
On the other hand, Japanese Patent Early-publication No. 60-96717 discloses an invention in which a shot peening is applied to the surface of a spring to generate compressive residual stress. Then a honing or sandblasting is applied to the surface to improve the surface roughness and increase the fatigue strength of the spring. However, this process also requires equipment of two devices. Thus the cost of equipment is high.
The present invention aims at providing a method and device for a shot-peening processing that can reduce operating costs and the cost of equipment while increasing the efficiency of treatment in the peening processing while achieving in a peening effect similar to the effect obtained by shot-peening processing plural times, that is, deeply generating compressive residual stress far from the top of the surface, while retaining the generation of the peak value of the compressive residual stress on the top of the surface.
The inventors of the present application extensively investigated to achieve the above object. As a result, they have found a method for shot-peening processing using shot in which two or three kinds of particles, each having an average particle diameter within the range of the predetermined average particle diameters, the ranges differing from one. another, and having a predetermined average particle diameter ratio to one another, are combined in a predetermined weight ratio, and a device for conducting this method.
Method 1 of the present invention is characterized by projecting, on a product to be subjected to peening, shot in which large-diameter particles having an average particle diameter of 300-1,000 xcexcm, and small-diameter particles having an average particle diameter of 20-300 xcexcm, the ratio of the average particle diameter of said small-diameter particles to that of said large-diameter particles being 1/3-1/15, are combined in a weight ratio such that the coverage of each of the particles is 100% or more in the same projection time.
Method 2 of the present invention is characterized by projecting, on a product to be subjected to peening, shot in which large-diameter particles, having an average particle diameter of 500-1,000 xcexcm, medium-diameter particles having an average particle diameter of 100-500 xcexcm, and small-diameter particles having an average particle diameter of 20-100 xcexcm, the ratio of the average particle diameter of said medium-diameter particles to that of said large-diameter particles and the ratio of the average particle diameter of said small-diameter particles to that of said medium-diameter particles each being 1/2-1/15, are combined in a ratio such that the coverage of each of the particles is 100% or more in the same projection time.
Also, the first device for a shot-peening processing to conduct the method of the present invention is characterized by connectively placing a classifying device at the bottom of a peening chamber provided with a projecting device at its top, by which classifying device shot that are worn and broken by their use into particles having a particle diameter other than the predetermined average particle diameters of the particles to be used are classified and removed while recycled shot are classified into particles of respective average particle diameters to be used (for above method 1, the large-diameter particles and the small-diameter particles, and for above method 2, the large-diameter particles, the medium-diameter particles, and the small-diameter particles), connectively joining the respective openings of the classifying device for discharging the large-diameter particles and the small-diameter particles to respective means for transferring shot, connecting the ends of the respective means for transferring shot to a tank for shot, in which tank a device for uniformly stirring and mixing is provided, and connectively joining the tank to the projecting device.
A second device for a shot-peening processing to conduct the method of the present invention is characterized by connectively placing a classifying device at the bottom of a peening chamber provided with a projecting device at its top, by which classifying device shot that are worn and broken by their use are classified into particles having particle diameters other than predetermined average particle diameters of the particles to be used and removed while recycled shot are classified into particles of respective average particle diameters to be used (for above method 1, the large-diameter particles and the small-diameter particles, and for above method 2, the large-diameter particles, the medium-diameter particles, and the small-diameter particles), connectively joining the respective openings of the classifying device for discharging particles of respective average particle diameters (for above method 1, the large-diameter particles and the small-diameter particles, and for above method 2, the large-diameter particles, the medium-diameter particles, and the small-diameter particles) to respective means for transferring shot, collectively connecting the ends of the respective means for transferring shot to respective supply pipes for the shot through respective flow control valves by which each flow rate is controlled to be at a predetermined weight ratio, and connectively joining the supply pipes to the projecting device.